1. Field of the Invention
The present invention relates to a flexible substrate in which a wiring conductor is arranged from one end to the other end and to an electronic device having this flexible substrate, for example, a portable terminal (cellular phone). More specifically, the invention relates to a foldable (opening/closing) electronic device which is repeatedly folded (closed) and deployed (opened) and to a flexible substrate for electrically connecting circuit substrates to each other which are arranged in a pair of casings in this electronic device.
2. Description of the Related Art
(Outline of Cellular Phone Structure)
FIG. 14 shows the schematic structure of a folding portable terminal S. In the portable terminal S, a first casing 200 and a second casing 300 are joined to each other through a hinge 400.
The first casing 200 has plural operation keys and others. A detachable battery (omitted from the drawing) and a circuit substrate 201 are placed inside the first casing 200. Mounted to the circuit substrate 201 are a tact switch, which operates in conjunction with manipulation of the operation keys, and other components.
The second casing 300 has a display. A circuit substrate 301 is placed inside the second casing 300. The circuit substrate 301 is mounted with parts relevant to the display.
(Structure of around Flexible Substrate)
The circuit substrate 201 and the circuit substrate 301 are electrically connected to each other through a flexible substrate 101. The two ends of the flexible substrate 101 are connected to connectors (not shown in the drawing) mounted to the circuit substrates 201 and 301, respectively.
The flexible substrate 101 is, as shown in FIG. 15, wound on itself (one roll) to form a tube-like shape at the middle. The rolled portion of the flexible substrate 101 is housed in the hinge 400, which is a pair of tubes 401 and 402 coupled to each other.
As shown in FIG. 16, the elongated flexible substrate 101 is composed of connection portions 110 and 111, straight portions 112 and 113 and a cranked portion 114, and is formed unitarily. The connection portion 110 is connected to the circuit substrate 201 shown in FIG. 15 whereas the connection portion 111 is connected to the circuit substrate 301 shown in FIG. 15.
Interposed between the connection portions 110 and 111 are the straight portions 112 and 113, which in turn sandwich the cranked portion 114. The cranked portion 114 is formed in a continuous manner between the straight portions 112 and 113.
The straight portions 112 and 113 are extended from the connection portions 110 and 111, respectively, in the longitudinal direction of the flexible substrate 101. The straight portions 112 and 113 are arranged in a staggered fashion in order to avoid overlapping each other when the flexible substrate 101 is bent in some way (rolled, for example).
In short, the cranked portion 114 is set diagonal with respect to the longitudinal direction of the flexible substrate 101 to reach the straight portions 112 and 113 both. As shown in FIG. 16, the plan view of the flexible substrate 101 therefore resembles the shape of the letter Z.
When the first casing 200 and the second casing 300 are opened and put in a deployed state as shown in FIG. 14 (when the second casing 300 is in the position indicated by the chain double-dashed line of FIG. 14), the cranked portion 114 of the flexible substrate 101 is rolled up in the hinge 400 shown in FIG. 15 by a degree not larger than 360° (one roll as shown in FIG. 15).
On the other hand, when the first casing 200 and the second casing 300 are closed and put in a folded state (when the second casing 300 is in the chain dashed line of FIG. 14), the cranked portion 114 of the flexible substrate 101 is rolled up by a degree between 360° and 540° (wound more than once) as shown in FIG. 17.
On one side of the flexible substrate 101, a power supply line 102 and plural signal lines 103 are arranged in the straight portions 112 and 113 and on the cranked portion 114 running between the connection portions 110 and 111 as shown in FIG. 16 (only three of the plural signal lines 103 are shown in FIGS. 16 and 18). The power supply line 102 and the signal lines 103 are formed from a wiring conductor (copper foil or the like).
The power supply line 102 and the signal lines 103 make the circuit substrate 201 and the circuit substrate 301 electrically conductive to each other. The power supply line 102 can have a desired width as long as the electric resistance of the power supply line 102 is equal to or lower than a predetermined level in bar of malfunction by voltage drop.
The signal lines 103 may be smaller in width than the power supply line 102 since the signal lines 103 only have to supply a current as an electric signal. In other words, the signal lines 103 can have a small volume in section since the lines have to carry only a small amount of current. The power supply line 102 is therefore set larger in width than the signal lines 103. The wiring conductor forming the power supply line 102 and the wiring conductor forming the signal lines 103 have the same thickness.
The plan view of the flexible substrate 101 has a cranked shape as shown in FIG. 16 because this enables the tubes 401 and 402 of the hinge 400 shown in FIG. 15 to house the cranked portion 114 of the flexible substrate 101 in a rolled up state. Another reason to give the flexible substrate 101 a cranked shape in plan view is, as well known, to avoid wire breakage of the power supply line 102 and the signal lines 103 in the cranked portion 114 of the flexible substrate 101 when the cranked portion 114 is rolled-up (see FIGS. 15 and 17).
The line width of the power supply line 102 is set uniform throughout the straight portions 112 and 113 and the cranked portion 114 as shown in FIG. 19 due to stabilize uniformity of the electric resistance. The power supply line 102 and the signal lines 103 in FIG. 18 and FIG. 19 are indicated by outlined areas.
To elaborate, a width W4 that is orthogonal to the slant of the cranked portion 114, which is slanted with respect to the longitudinal direction (roll-up direction) of the flexible substrate 101, is equal to a width W2 and a width W3, which denote the widths of the straight portions 112 and 113, respectively (W4=W2=W3).
Similarly, a line width K4 of a portion of the power supply line 102 that is orthogonal to an oblique portion of the power supply line 102 that is in the cranked portion 114 is equal to line widths K2 and K3 of portions of the power supply line 102 that are in the straight portions 112 and 113 (K4=K2 =K3)
A width W1 of the cranked portion 114, which is slanted with respect to the longitudinal direction (roll-up direction) of the flexible substrate 101, is orthogonal to the roll-up direction and is wider than any of the widths W2 and W3 of the straight portions 112 and 113 and the width W4 (W2=W3=W4 <W1).
Therefore, as shown in FIG. 19, the cross-sectional area of the straight portions 112 and 113 of the flexible substrate 101 in the direction orthogonal to the roll-up direction differs from the cross-sectional area of the cranked portion 114 of the flexible substrate 101 in the direction orthogonal to the roll-up direction; the cranked portion 114 is larger in cross-sectional area than the straight portions 112 and 113.
In the straight portions 112 and 113, the roll-up direction of the flexible substrate 101 matches the wiring conductor line direction (the direction in which the power supply line 102 and the signal lines 103 run). In the cranked portion 114, on the other hand, the roll-up direction of the flexible substrate 101 does not match the wiring conductor line direction since the cranked portion 114 is slanted with respect to the longitudinal direction of the flexible substrate 101.
This makes a width K1 of the power supply line 102 in the cranked portion 114 different from the widths K2 and K3 of the power supply line 102 in the straight portions 112 and 113 and from the width K4 as described above; the width K1 of the power supply line 102 in the cranked portion 114 is wider than any the widths K2 and K3 of the power supply line 102 in the straight portions 112 and 113 and the width K4 (K2=K3=K4<K1).
The flexible substrate 101 in the hinge 400 shown in FIG. 15 is therefore more rigid in the cranked portion 114 than in the straight portions 112 and 113. In short, the cranked portion 114 has higher rigidity than the straight portions 112 and 113.
With the rigidity varied from one portion of the flexible substrate 101 in a wound (rolled up) state to another, the flexible substrate 101 cannot be curved uniformly without taking some measure to do so. When the flexible substrate 101 is wound (curved), a force proportionate to the radius of curvature is applied to the cranked portion 114 of the flexible substrate 101 as shown in FIG. 20. The reaction force of the flexible substrate 101 is determined by the widths W1 through W3 of the flexible substrate 101 and the line widths K1 through K3 of the power supply line 102 (and the line width of the signal lines 103).
The reaction force is larger in the cranked portion 114 of high rigidity than in the straight portions 112 and 113, and the stress centers around the borders between the cranked portion 114 and the straight portions 112 and 113. This is because the radius of curvature is large in the cranked portion 114 of high rigidity and becomes small in the straight portions 112 and 113 of low rigidity, creating extremely large flexures at the borders between the cranked portion 114 and the straight portions 112 and 113.
In FIG. 21 which shows the flexible substrate 101 in a rolled up state, a compressive force works on the surface of the power supply line (wiring conductor) 102 which is on the inner circumference side whereas a tensile force works on the interface between the power supply line (wiring conductor) 102 and a base 104. This applies a bending force to the wiring conductor 102 and accordingly the wiring conductor 102 receives a squashing force from the direction of the base 104, which is made of synthetic resin.
As shown in FIG. 18, a bending moment acts on the flexible substrate 101 in a border portion 115, which borders the straight portion 112 and the cranked portion 114, and in a border portion 116, which borders the straight portion 113 and the cranked portion 114, thereby causing the stress to center on the border portions 115 and 116. This stress concentration is particularly prominent in the power supply line (wiring conductor) 102 whose wideness makes the rigidity difference between the cranked portion 114 and the straight portions 112 and 113 larger.
The cranked portion 114 of the flexible substrate 101 is housed in the hinge 400 as shown in FIG. 15 while rolled up to such a curvature that does not break the wiring conductors (the power supply line 102 and the signal lines 103 in FIG. 19). As the first casing 200 and the second casing 300 of the portable terminal S are repeatedly deployed (opened) and folded (closed), fatigue from the repetitive opening and closing accumulates in portions of the wiring conductors 102 and 103 where the stress centers. The accumulated fatigue causes cracks 120 (see FIG. 21), which eventually break the wiring conductors 102 and 103. The cracks 102 tend to appear more often in the wiring conductor that forms power supply line 102 since the line width (K1 through K3) of the power supply line 102 is wider than the line width of the signal lines 103.
JP 2002-158458 A discloses a structure that has been designed to solve the above problem. In this structure, slits are opened in the base of the flexible substrate to thereby ease the stress working on the wiring conductors and prevent the wiring conductors from breaking.
The structure according to JP 2002-158458 A needs to add a step of opening the slits in the base to a common flexible substrate manufacturing process. If the slits are to be formed after the wiring conductors are arranged on the flexible substrate, the base cutting work for opening the slits requires carefulness to avoid cutting the wiring conductors by mistake.
On the other hand, if the slits are to be formed before the wiring conductors are arranged on the flexible substrate, the wiring conductors have to be placed between the slits. The structure according to the patent application cited above thus complicates the flexible substrate manufacturing process and lowers the productivity of the flexible substrate.
Furthermore, the widths of the wiring conductors are limited by the interval between the slits in the invention disclosed in the above patent application. The limitation is inconvenient when the width of the power supply line (wiring conductor) which is set such that the current resistance of the power supply line does not exceed a predetermined level has to be changed (increased) in order to prevent power consumption from exceeding a predetermined level.
Since the slit interval limits the width of the power supply line (wiring conductor) to a given width or smaller in the structure according to the patent application cited above, the permissible current is exceeded making it impossible to flow an appropriate current in the wiring conductor.
Moreover, in the invention according to the cited patent application, the direction in which the hinge rotates (the direction indicated by the arrow F in FIG. 8 of JP 2002-158458 A) is orthogonal to the direction in which the slits run (the direction indicated by the arrow E in FIG. 8 of JP 2002-158458 A).
This means that the maximum width of the flexible substrate is limited by the internal diameter of the hinge, and that the flexible substrate cannot have a necessary number of signal lines when the width of the power supply line is set large.
When there are many signal lines, on the other hand, the permissible current per signal line is small. Another problem of the structure according to JP 2002-158458 A is that it is difficult in practice to form slits, each of which has to be 1 mm or more in width, in manufacture of the flexible substrate whose width is limited to the inner diameter of the hinge.
The stress working on the wiring conductor (power supply line) 102 can be reduced by reducing the wiring conductor width throughout the entire length of the wiring conductor 102 (by equally reducing the line widths K1 to K3 shown in FIG. 19). With the stress decreased, fatigue brought by the repetitive deploying and folding described above is prevented from accumulating.
In recent years, portable terminals are demanded to have various advanced functions. The folding portable terminal S is not an exception and is receiving the same demand for more advanced functions.
The folding portable terminal S is therefore required to form wiring conductors (the power supply line 102 and the signal lines 103) at a density higher than the past (to form more of the lines) on the flexible substrate 101, which electrically connects the circuit substrate 201 in the first casing 200 and the circuit substrate 301 in the second casing 300 to each other.
However, increasing the density of the wiring conductors 102 and 103 on the flexible substrate 101 leads to the difference between the rigidity of the cranked portion 114 and the rigidity of the straight portions 112 and 113 and, accordingly, to larger flexures in the border portions 115 and 116.
Also, in order to obtain various advanced functions, it is necessary to supply a large power from the circuit substrate 201 in the first casing 200 to which a battery (power supply pack) is mounted to the circuit substrate 301 in the second casing 300.
A reduction in wiring conductor width throughout the entire length of the power supply line 102 (a uniform reduction of the line widths K1 to K3) therefore raises problems such as an increase in current resistance of the power supply line 102 which leads to malfunction by voltage drop.
In conclusion, the structure according to JP 2002-158458 A is not suitable for portable terminals which are demanded to have advanced functions.
The flexible substrate according to JP2002-158458 A cannot provide advanced functions because its insufficient durability against repetitive deploying and folding (bending) of the portable terminal.